This invention relates to a circuit for driving an electroluminescent ("EL") device, and more particularly to a drive circuit which charges an EL device during one part of a power cycle and recovers EL device discharge energy during another part of the power cycle. The recovered energy then is re-applied to charge the EL device.
A typical electroluminescent device is a flat-panel display. The panel is formed by an electroluminescent layer sandwiched between a pair of outer conductive layers. The electroluminescent layer is formed by a ceramic dielectric having an electroluminescent phosphor coating. The outer conductive layers form horizontal and vertical electrodes on opposite sides of the dielectric. Activation of crossing electrodes illuminates the intersected phosphor area to light a pixel on the display.
In operation a pixel is activated by repetitive charging and discharging of the panel area defining the pixel. The panel in essence is a capacitive device typically modelled as a capacitor having a nonlinear loss element. The loss element varies with voltage and switching frequency. Conventional electroluminescent devices lacking energy recovery schemes consume large amounts of energy through repetitive charging and discharging and exhibit relatively low power conversion efficiency.
During a charging phase the panel absorbs energy as capacitance. Then during a discharge phase, the capacitive energy is discharged through a resistive device to be dissipated in the form of heat. During the charging phase, only a very small portion (e.g., less than 5%) of the energy delivered to the panel is expended in the creation of light. Most of the energy is stored in the panel capacitance or dissipated in the charging path resistance as heat. Accordingly there has been a need to improve the efficiency of electroluminescent panels and reduce power consumption.
Schemes for improving the efficiency include energy saving schemes and energy recovery schemes. Energy saving schemes have been directed toward shaping the charging current so as to reduce the energy lost as heat through the charging path resistance. This invention is directed toward a drive circuit employing an energy recovery scheme.
According to energy recovery schemes, energy discharged by the EL device is forced back into a system power source. To do so a portion of the discharged energy typically dissipated through a resistive device, instead, is directed elsewhere for storage and re-use. Prior energy recovery schemes have directed the energy into a "resonant tank" circuit. The EL device discharged in one burst based on the load capacitance timing constant. A drawback of such approach is that large components are required to move and capture the energy. Accordingly there is a need for an alternative energy recovery scheme in which smaller components can be used. Smaller low-profile components are desirable for implementing a drive circuit with power recovery in hand-held EL devices.